How did the dinosaurs evolve into such big creatures?
from Curdworth, age 7, ?, ?, UK; July 11, 2001

TOM: The ancestors of dinosaurs and the early dinosarus were all small creatures (Eoraptor, the most primitive dinosaur, was only about 3 feet long, and half of that was tail). However, all dinosaurs had their hindlegs directly under their body. This probably evolved originally for speed, but it left them in (literally) a great position to get bigger, because legs directly underneat their body could support larger and larger sizes.

Dinosaurs may have increased in size for various reasons. In the case of plant-eaters, they could reach higher then other animals if they got larger, and they were better protected from attack if they got larger. For meat-eaters, being larger meant that you could kill more different kinds of creatures, and also chase away smaller predators from their kills.

One thing that may have helped dinosaurs get bigger than land mammals did later was that dinosaurs were all egg layers. Modern placental bodies have to keep their young inside their body, and the length of this time (the gestation period) gets longer as mammals get bigger. Mice can have many babies a year, cats and dogs fewer, humans fewer still, and elephants have gestation periods of 22 months! A placental mammal the size of Brachiosaurus would have gestation periods of four or five years: too long to be pregnant!!

Even the biggest dinosaur, on the other hand, could lay a dozen or more eggs every single year, so being big wasn't a strain on dinosaur parents.